Exemplary embodiments of the present invention relate to a drive circuit of a synchronous rectifier, and more specifically, to energy feedback in a current-driven synchronous rectifier (SR) designed for a voltage doubler rectifier.
With the development of semiconductor technology, the on-resistance of low and middle voltage MOSFET has decreased. In order to reduce conduction loss, a synchronous rectifier (SR) is used in low voltage high current switching power applications. In some low and middle voltage DC-DC and AC-DC applications, the output voltage could be 48 volts (V) and/or higher. If a half-wave rectifier or a center-tapped rectifier is used in such high voltage applications, the voltage stress of the rectifier is too high to use low voltage rectifiers to reduce the conduction loss. When the voltage of a MOSFET is above 200V, both the cost of the MOSFET and the on-resistance increases, and thus is not suitable for synchronous rectification applications. Therefore, for a conventional rectifying circuit, when the output voltage of a rectification circuit is above 60V, normal diode rectification is usually adopted. However, there could be certain oscillation between a transformer leakage inductance and an intrinsic capacitance in circuit. The oscillation may cause a voltage spike and as a result, the voltage stress of the rectifying components may be much higher than that in the ideal conditions.
A voltage doubler rectifier is able to eliminate a voltage spike across the rectifying components and reduce the voltage stress, by clamping the withstanding voltage of the rectifier at the same level of the output voltage. Therefore, a low voltage MOSFET can be used for synchronous rectification to reduce the conduction loss. However the conventional method of driving a MOSFET by sensing current signal requires a current transducer (CT) with large turns in its windings. In particular, when the output current is very high, there will be too much energy transferred to the drive circuit. If this part of redundant energy could not be fed back to the source of the output voltage, there would be additional loss. In light of the particularity of the voltage doubler rectifying circuit, one of the SR drive circuits needs to be ground-floated. So when using traditional current-driven rectifiers, it's impossible to have both SR's driving energy fed back to the output.